The present invention relates generally to signal phase detection, and more specifically to apparatus and techniques employing quadrature correlation to determine the phase angle between two signals.
The relative phase between two periodic signals, one of which may be a reference signal, is utilized in a variety of apparatus for communication, testing and evaluation, object detection and/or object location, as well as for the other purposes. Both analog and digital techniques are known for accomplishing phase measurements. Where noise free and undistorted signals are involved and the phase angles to be measured vary over a limited range of angles, the measurement may be made with relatively simple apparatus. However, an increasing number of requirements exist for apparatus capable of reliably and accurately measuring the phase of signals which are not free of noise and/or distortion, and of measuring phase angles over a range of up to 360.degree..
Measurement of phase over an extended range of phase angles by means of many prior art technqiues may result in ambiguities unless special provisions are made for determining which quadrant or 180.degree. sector contains the angle of interest. Such special provisions frequently comprise separate circuitry for making "coarse" and "fine" determinations which may establish the sector and the magnitude of the angle within that sector respectively. Noise tends to introduce errors into phase measurements. To comply with more demanding requirements, it is often necessary to employ averaging and/or filtering to minimize errors resulting from noise and/or distortion.
Typical prior art circuits for providing phase measurements over an extended range of phase angles are shown in U.S. Pat. Nos. 3,286,176, 3,559,161 and 3,663,956 issued to M. H. Birnboim, I. G. Raudsep and B. W. Purdy et al. on Nov. 15, 1966, Jan. 26, 1971 and May 16, 1972 respectively. In each of these systems sinusoidal input signals are converted to square wave signals. A timing interval is initiated at a detectable reference time and utilized to provide a coarse phase angle determination. A counter is employed to monitor a higher repetition rate clock pulse train during the coarse timing interval to provide a fine phase angle determination.
More recent techniques for accomplishing phase angle measurement over an extended range of phase angles are shown in an article entitled "Edge-triggered flip-flops make 360.degree. phase meter" in the Aug. 21, 1975 issue of "Electronics" on pages 100 and 101, and in U.S. Pat. No. 3,906,361 issued to N. Nessler et al. on Sept. 16, 1975. The technique disclosed in the "Electronics" article is based on the assumption that an arbitrarily chosen one of two input signals lags the other. It utilizes a pair of interconnected edge triggered flip flops to generate a saw tooth waveform which increases from zero to a maximum value as the phase difference increases from zero to 360.degree.. The technique employed in the Nessler et al. patent utilizes information concerning the first to occur of negative going zero crossings of the input signals in addition to the angular difference between zero crossings to unambiguously determine phase angles from -180.degree. to +180.degree..
It is known that digital systems in general have certain inherent advantages over analog systems. Some of these are set forth in U.S. Pat. No. 3,039,094 issued to V. C. Anderson on June 12, 1962 which discloses a digital system for beam steering of a fixed transducer array. One disclosed embodiment utilizes a shift register memory apparatus to accomplish beam steering.
Among the general advantages set forth for digital systems are that the digital signals produced thereby and used therein are directly compatible with digital computers, which provide great flexibility and signal processing power. Also, the use of digital signal processing provides for a normalized output characterized by a true signal-to-noise ratio rather than proportionality to either signal or noise alone. This normalization reduces the dynamic range requirements of components and circuits used in the system, since variations in background noise do not change the reference noise output of the system. Further, digital systems are minimally susceptible to errors and changes in calibration caused by aging of components and changes in operating parameters.
As also discussed in U.S. Pat. No. 3,039,094, in many situations the polarity of a band limited signal contains nearly as much information as the complete analog signal itself. This principle may be advantageously applied by dividing an input analog signal of interest into two classes determined by its instantaneous polarity, and representing it by a time series consisting of two possible voltage states. This so called "clipped signal" may be simply and conveniently produced by a clipper or clipping amplifier. The clipping level may be set to a level desirable for subsequent digital signal processing, and the voltage states assigned values of +1 and -1.
U.S. Pat. No. 3,039.094 further states that, "a band limited signal may be represented by a sequence of individual amplitude samples providing the sampling rate is equal to or greater than twice the highest frequency in the signal." This principle and the principle of the preceding paragraph can be combined to permit the input signal to be represented by a set of binary digits, each sample having a value of +1 or -1, depending on the polarity of the input signal at the sampling instant.
It is also known that signal correlation techniques may be advantageously utilized to overcome detrimental effects of noise and/or distortion in the input signals. Both analog and digital forms of signal correlation are known. For example, U.S. Pat. No. 3,346,862 issued to I. G. Raudsep on Oct. 10, 1967 discloses an analog autocorrelation system for determining the time difference between a pair of pulse signals of common origin. The system employs weighting filter means for modifying the power spectra of the pulse signals to optimize the autocorrelation function. U.S. Pat. No. 3,646,334 issued to Ivar Wold on Feb. 29, 1972 discloses a hybrid analog/digital system in which two input signals to be correlated are sampled, the samples of one of the signals inserted into a recirculating memory time compressor, the output of the memory multiplied with the other signal, and the product signal averaged to determine the correlation of the input signals.
Other known refinements in correlation techniques involve multiplication of the input signal with each of quadrature components of a reference signal. The product signals are integrated with respect to time to produce real and imaginary components of correlation of the input and reference signals. The real and imaginary components are combined in accordance with the Pythagorean theorem to produce an indication of correlation of the signals. A variation of this method is embodied in a signal processor disclosed in U.S. Pat. No. 3,878,526 issued to N. E. Pedersen on Apr. 15, 1975. As is typical of traditional quadrature demodulation systems, the Pedersen processor involves an analog implementation.
The applicants have uniquely combined the advantages of quadrature correlation and digital signal processing utilizing clipped signals to provide a clipped quadrature correlation phase determining method and apparatus. The method and apparatus provide exceptionally high immunity to errors introduced by noise and distortion, and permit reliable and accurate determination of phase angles throughout a 360.degree. range through the use of a unique and simple algorithm which may be executed with minimum computational capabilities.